1. Field of the Invention
This invention describes a method, an apparatus, and a system for remotely adjusting by mechanical and electronic means the plumb-to-level and the compass heading of one or a plurality of communication antennas. The term plumb-to-level will be used throughout this description to represent absolute measurements with respect to true vertical. And, the term compass heading will be used throughout this description to represent absolute compass heading direction with respect to magnetic North.
A continuing problem for cellular telephone network planners is that of base station over or under coverage. That is, if the overlapping area between two cells is too large (i.e., over coverage), increased switching between the base station (handoff) occurs, which strains the system. Likewise, if the overlapping area between two cells is too small (i.e., under coverage), gaps in service, or nodes, will occur. There may even be interference with other cellular networks using the same, or nearby, operating frequencies. To minimize the over and under coverage effects, a cost effective means to precisely position the antenna remains a continuing challenge.
This invention is not limited to antennas for cellular telephone network use only, but since this is the largest use, we will use this application in the following description. In general, radio frequency antennas are described as having a radiation pattern that is referred to as being a horizontal pattern and a vertical pattern, with the former being referenced along the horizon, as would a compass heading, and the latter being referenced from the vertical, as would plumb-to-level. Since cellular telephone traffic tends to concentrate in certain areas such as along a busy highway, further performance optimization is accomplished by the ability to precisely position the antenna in a concentrated area.
The industry term for antenna position with respect to vertical angle is down-tilt. The term for antenna position with respect to horizontal angle is azimuth. Measurements of plumb-to-level (P-L) and compass heading (CH) are absolute and are referenced to the earth itself. Current methods for obtaining antenna settings such as down-tilt angle are measured with respect to a part of the tower itself. In the case of most radio antennas, this measurement is made with respect to the tower. However, these tower referenced measurements are subject to many induced errors caused by weather, ground shifting, disturbances, or human error that is inherent to the measurement process itself. Once the reference is flawed, then all the calibrations based upon the reference are in error.
There are several ways to adjust antenna down-tilt. One way is to adjust it electronically by using a phased-array antenna. Another way is by mechanical means, as in using a special down-tilt mounting bracket such as the EZ-Tiltz™ bracket. The mechanical method is the simplest method since it does not require sophisticated timing and electronic phasing circuits. A third way to adjust the antenna down-tilt is to use closed loop electromechanical control devices using encoders. Because of the reference issue described above, this method is also flawed, and care must be taken to use components that are compatible with electromagnetic interference (EMI) sensitive communication electronics. The use of high frequency devices such as stepping motor drives is not recommended.
Unfortunately, antenna optimization that is accomplished by solely adjusting the down-tilt alone is limited. Improvements made by adjusting the down-tilt are only valid for one direction of the horizontal radiation pattern. Within the most critical range of down-tilt, the actual radiation coverage varies more according to the azimuth direction, but demonstrates that both the down-tilt and the azimuth adjustments are integral. A change results in a horizontal radiation half-power beam width which gets broader with increasing down-tilt angle rather than the desired narrower, more focused radiation beam. Since azimuth adjustments for antenna sectors (more than one antenna acting as one antenna) are difficult to adjust manually and are not available electronically, site planning personnel to date have not been able to accurately compensate for this effect. Site surveys provide P-L and CH information. Until now, only fixed, manual adjustments referenced to points on the tower are assumed to be accurate. This invention allows remote adjustments with absolute reference to survey data.
Cellular telephone network antenna systems found in the center of a “cell” usually consist of three sectors, positioned at 120° segments of the complete circle. Each sector usually consists of four antennas mounted on a common mounting bar. From this, it can be seen that a typical cellular telephone antenna site can have up to twelve antennas needing periodic adjustment. This is very labor intensive and expensive, and usually involves dangerous work high above the ground.
2. Description of Related Art
Singer et al., U.S. Pat. No. 6,239,744 B1, teaches a method for adjusting antenna down-tilt, but only from a broad-brush perspective. Singer fails to address the need for azimuth adjustments in order to optimize beam coverage of a specific location when consideration is given to traffic patterns, topography, and other networks. Further, the Singer patent is based on the use of built-in controllers for each antenna and antenna sector, remembering stored data, and utilizing local and remote displays. In Singer, sensing down-tilt position of the antenna is left to an angle decoder to determine the angle between the antenna and its mounting structure, but does not address the need to coincide site survey data and actual site conditions. Much attention is given to operational function in Singer, but little attention is given to indicate how the actual hardware, or any integrated system, may be created by following its teachings.
Zimmerman et al., U.S. Pat. No. 6,232,928, Bernier, U.S. Pat. No. 5,029,179, and Chavez, U.S. Pat. No. 5,963,179, all teach manually adjusted down-tilt or azimuth antenna brackets. No mention of a remote means of adjustment is made.
Fulop, U.S. Pat. No. 5,583,514, teaches a satellite antenna position optimization system that is fast, complicated and expensive, suitable for government satellite tracking, but not suitable for low cost commercial installations such as that required by small cellular antenna sites. Fulop teaches a method of using GPS data to establish antenna position, which is outside the scope of this invention.
Hill, U.S. Pat. No. 5,461,935, teaches a slip clutch linear actuator. This design is fatally flawed because over travel of the actuator could cause the mechanism to bind. With the spring loaded clutch, actuators are limited to a fixed amount of torque. If this torque limit is exceeded, the drive reaches a point of slippage, thereby causing an irreversible jamming condition due to limited torque settings of the slip clutch.